David Cameron knows that as matters stand the Commons is unlikely to vote to arm elements of the Syrian opposition. This might be different were Barack Obama to line up behind Britain and France's desire to arm such elements, as he eventually lined up behind them over military intervention in Libya. But in any event, American intervention, in the form of providing anti-tank and anti-aircraft missiles as well as training people to using them, could tip the military balance within Syria against Assad, which the Prime Minister obviously favours. So Cameron won't mind Putin's brutal treatment of him this morning (indeed, he will more worried by the pronouncement on Syria today of a non-Russian Boris) if he gets the outcome he wants from Obama: namely, American arming of part of that Syrian opposition. This morning's exchanges are part of a diplomatic dance in which the main performers are America and Russia.

The choice facing the western world has no upside. On the one hand, Obama and his allies don't arm the opposition, in which case America walks a bit smaller in the world, and Russia a bit taller – a reminder that non-intervention by the United States as well as intervention has a political and diplomatic cost. Or on the other, America gets suckered into a proxy war with Russia over Syria – one in which victory could only be assured by first arming factions of the opposition, then training them, then backing them up with air power and ground troops if necessary and then, most likely, governing the country in some form (and all, remember, without a U.N mandate). Obama and Cameron are doubtless gambling that arming those factions – or threatening to – will drag Assad to the peace table, but how likely is this to happen? Even if it did, would British troops not be called upon to play a part in policing an unstable and fissile settlement (at a time when they are more stretched than ever)? And would war-torn Syria, in such circumstances, be more likely to become a liberal democracy or a sectarian state? I repeat: Britain has no national interest in intervening on the Saudi side of what increasingly is a war between salafist and Shi'ite Islam.